1. Technical Field
The invention relates generally to a camera system configured to detect infrared radiation such as, for example, the identification of various substances. More specifically, the invention relates to a camera system configured to detect a limited spectral bandwidth of infrared radiation.
2. Description of the Related Art
“Leak detection and repair” (LDAR) is a common problem in commercial applications where various substances are processed, stored, distributed, and utilized. In the petrochemical industry, leak detection devices include sniffers, scanners and passive imaging devices configured to identify a petrochemical leak by sensing the absorption of infrared radiation by the leaking compound at one or more predetermined infrared absorption bandwidths. In particular, methane (CH4), has strong infrared absorption bands approximately centered at the non-visible wavelengths 1.33 μm, 1.67 μm, 3.3 μm and 7.6 μm, and it is known to construct leak detecting devices to determine if methane is present in a gas sample by determining if the gas sample absorbs radiation at one or more of the methane absorption wavelengths. Similarly, other compounds may be detected by leak detection devices tuned to determine if other compounds are present in a gas sample by determining if the gas sample absorbs radiation at one or more absorption bands associated with the other compounds.
One example of a sniffer device is disclosed in U.S. Pat. No. 7,022,993 to Williams II et al. The sniffer device draws a gas sample into a chamber through a probe, transmits an infrared radiation beam through the gas sample to a photo detector, and a photo detector response signal is used to determine if the gas sample is absorbing infrared radiation at one or more predetermined absorption bands. One problem with using a sniffer device to detect gas leaks is that the probe must take in a gas sample directly from the leak plume in order to detect the leak. Accordingly, in a large facility or along miles of distribution conduits, leak detection by using a sniffer device is often inefficient and unreliable because leaks may be missed. Moreover, a user must be able to place the probe in the leak plume and this may not always be practical.
One example of a scanner device, called a laser methane detector, is disclosed in U.S. Pat. No. 7,075,653 to Rutherford. The laser methane detector scans a survey area with a tunable IR laser diode emitter and analyzes IR radiation reflected back from the survey area to a photo detector. If the presence of a methane plume is detected in the survey area, the laser methane detector alerts an operator by sounding an audible alarm. The tunable IR laser diode emitter is tuned over a range of wavelengths that includes in-band wavelengths, (approximately 1.67 μm), that are absorbed by methane, and out-of-band wavelengths that are not absorbed by methane and to use the photo detector response to determine if methane is present. The laser methane detector provides an advantage over a sniffer because the laser methane detector may detect a methane gas plume from a remote distance. However, one problem with the laser methane detector disclosed by Rutherford is that the tunable IR laser emitter is limited to emitting over a wavelength range of about 1.2-2.5 μm. Accordingly, the laser methane detector is only usable to detect compounds with a strong absorption band within the wavelength range of about 1.2-2.5 μm.
One example of a passive imaging device configured to detect the presence of methane and other hydrocarbon gas plumes is a video thermography camera that includes a lens positioned to form an image of a survey scene that may contain an infrared absorbing gas plume. The image of the survey scene is focused onto a focal plane array and an optical band pass filter is positioned between the lens and the focal plane array to limit the spectral bandwidth of the image of the survey scene to a desired wavelength range. The desired wavelength range corresponds with an absorption band of a compound that it is desired to detect in the image of the survey scene.
The example thermography camera includes a focal plane array that comprises indium antimonide (InSb) IR photo sensor elements. InSb photo sensor elements have a usable responsivity over the approximate spectral range of 1-5.5 μm, but are more practically limited to a usable range of 3.0-5.0 μm. Accordingly, the example thermography camera is practically limited to detecting leak plume containing compounds that have absorption bands in the spectral range of 3.0-5.0 μm. While that range is suitable for detecting methane and other hydrocarbon compound leaks, there is a need for a thermographic leak detector that operates to detect compounds having absorption bands above 5.0 μm.
Another problem with conventional thermographic leak detection systems is that InSb focal plane arrays have a broad spectral responsivity, e.g. 2 μm, as compared to typical absorption bands, which may have a spectral bandwidth of 0.1-0.3 μm. The problem is that the extra spectral responsivity range of the InSb focal plane arrays contributes dark current signal noise that ultimately reduces the contrast of the leak plume as compared to the background of a video survey image. Accordingly, it is desirable to use a photo sensor that has a spectral responsivity range that is spectrally tuned to the absorption bandwidth of the compound to be imaged in order to increase image contrast.
Additionally, in other industries, notably electrical power distribution, there is a need for a thermography camera for detecting leaks of the industrial gas sulfur hexafluoride (SF6). SF6 is commonly used as an electrical insulator and has a strong absorption band at approximately 10.57 μm. Conventional thermography cameras do not have a focal plane array capable of forming an image of a survey scene over a wavelength range that includes 10.57 μm.
There is also a need for a thermography camera for detecting other gasses associated with other particular wavelength ranges. In this regard, the various aspects of conventional thermography cameras described above may similarly limit the ability of such cameras to detect gasses in these other particular wavelength ranges.